1. Field of the Invention
The present invention relates to diodes, and more particularly, to light emitting diodes (LEDs). Although the present invention is discussed with reference to light emitting diodes, the present invention can be used in a wide range of applications including, for example, other types of diodes such as laser diodes.
2. Discussion of the Related Art
Gallium-Nitride (GaN) based opto-electronic device technology has rapidly evolved from the realm of device research and development to commercial reality. Since they have been introduced in the market in 1994, GaN-based opto-electronic devices have been considered one of the most promising semiconductor devices. The efficiency of GaN light emitting diodes (LEDs), for example, has surpassed that of incandescent lighting, and is now comparable with that of fluorescent lighting.
The market growth for GaN based devices has been far exceeding than the industrial market prediction every year. In some applications, such as traffic lights and interior lighting in automobiles, the low maintenance cost and reduced power consumption of GaN LED's already outweigh the relatively high manufacturing costs. In other applications such as general room lighting, manufacturing costs are still much too high, and a simple economy of scale reveals that such devices are not yet the solution. Although considerably more demanding of materials quality and device design, room temperature, continuous wave blue lasers with reasonable lifetimes have been demonstrated. Their continued development combined with the potentially high-volume market should bring costs to acceptable levels, provided that they can be manufactured with high yield. GaN-based high-power electronic devices should also find application in mobile communications, another high-volume market. In order to expand the current AlInGaN-based LED market, it is crucial to develop low cost processing techniques without sacrificing device performances. Moreover, high power optical devices are strongly needed to replace the light bulb lamps. Accordingly, two important technical issues need to be solved at the same time, i.e., economical device production and high output power device fabrication.
Outdoor signboard display has been one of the primary markets since the introduction of blue LEDs. In such application, the light output is considered one of the most important device parameters in AlInGaN-based LEDs. As a result, the unit device price is approximately proportional to the light output intensity. Moreover, recently, the white LED application requires higher light output than currently available to replace the incandescent light bulbs for illumination. Therefore, developing a technology to increase light output is one of the most important tasks in the AlInGaN-based opto-electronic devices.
FIG. 1 shows a conventional light emitting diode structure. The conventional LED includes a substrate 10, such as sapphire. A buffer layer 12 made of, for example, gallium nitride (GaN) is formed on the substrate 10. An n-type GaN layer 14 is formed on the buffer layer 12. An active layer such as a multiple quantum well (MQW) layer 16 of AlInGaN, for example, is formed on the n-type GaN layer 14. A p-type GaN layer 18 is formed on the MQW layer 16.
The MQW layer emits photons “hv” in all directions to illuminate the LED. FIG. 1 shows directions 1, 2 and 3 for convenience. Photons traveling in directions 1 and 2 contribute to the intensity of the LED. However, photons traveling in direction 3 become absorbed by the substrate and the package which house the LED. This photon absorption decreases the light extraction efficiency resulting in reduced brightness of the LED.
There are two main methods to increase light output of the AlInGaN-based LEDs. The first method is to improve external quantum efficiency of the LED device by epitaxial growth and device structure design. This technique requires high quality epitaxial growth techniques that include MOCVD (Metal Organic Chemical Vapor Deposition), MBE (Molecular Beam Epitaxy), and HVPE (Hydride Vapor Phase Epitaxy) and sophisticated device design. In particular, MOCVD has been the most common growth tool to grow commercial grade AlInGaN-based LEDs. It is generally known that the epitaxial film quality is strongly dependent on the types of MOCVD growth method. Hence, in the manufacturing point of view, it is more difficult to improve optical light output of the LED devices by such growth technique.
Another method to enhance the optical light output is increasing the light extraction efficiency by optimizing the LED chip design. Compared to the method of increasing external quantum efficiency by epitaxial growth and device structure design, this method is much simpler and easier to increase the light intensity of the LED device. There have been many attempts to design the most efficient device design. However, thus far, these attempts have not led to the level of efficiency and brightness desired from the diode. Moreover, existing designs require high manufacturing cost. Accordingly, a diode is needed that has high brightness capability, an efficient design and low manufacturing cost.